Initial colonization of tooth surfaces by oral viridans streptococci, including Streptococcus gordonii, leads to the eventual formation of biofilms called dental plaque. The most common disease of man, caries and periodontal disease, result from imbalances in the oral microflora, which allow pathogenic species to dominate. Initial plaque formation is characterized by the adhesion of planktonic cells of bacteria such as streptococci to tooth surfaces via specific salivary proteins of the acquired pellicle. Subsequent growth of these initial colonizers and other bacteria on the abiotic surface leads to the formation of dental plaque. Studies have extensively characterized of the initial binding of the planktonic bacteria to saliva-coated/hydroxyapatite surfaces, but the prerequisite signals that trigger the transition from a planktonic to a sessile mode of life and the subsequent accumulation of dental biofilms are poorly understood. It is hypothesized that novel genes are required for initial dental biofilm formation, and identification of such genes and characterization of their expression will be crucial for the development of novel methods of dental plaque control. A simple, but effective method of microbial accumulation on polystyrene surfaces will be used in this study to characterize isolation of biofilm-defective mutants of S. gordonii using Tn916 transposan mutagenesis, (2) characterization of biofilm-defective mutants, and (3) cloning and genetic analyses of biofilm genes. These studies of biofilm formation in streptococci will provide valuable information on the initial stages of dental plaque formation. Understanding the mechanisms involved in biofilm formation will be crucial for the development of novel therapeutic strategies to modify the composition of dental biofilm flora towards that found in health.